Metabolomic and transcriptomic analysis reveals endogenous substrates and metabolic adaptation in rats lacking Abcg2 and Abcb1a transporters.

Abcg2/Bcrp and Abcb1a/Pgp are xenobiotic efflux transporters limiting substrate permeability in the gastrointestinal system and brain, and increasing renal and hepatic drug clearance. The systemic impact of Bcrp and Pgp ablation on metabolic homeostasis of endogenous substrates is incompletely understood. We performed untargeted metabolomics of cerebrospinal fluid (CSF) and plasma, transcriptomics of brain, liver and kidney from male Sprague Dawley rats (WT) and Bcrp/Pgp double knock-out (dKO) rats, and integrated metabolomic/transcriptomic analysis to identify putative substrates and perturbations in canonical metabolic pathways. A predictive Bayesian machine learning model was used to predict in silico those metabolites with greater substrate-like features for either transporters. The CSF and plasma levels of 169 metabolites, nutrients, signaling molecules, antioxidants and lipids were significantly altered in dKO rats, compared to WT rats. These metabolite changes suggested alterations in histidine, branched chain amino acid, purine and pyrimidine metabolism in the dKO rats. Levels of methylated and sulfated metabolites and some primary bile acids were increased in dKO CSF or plasma. Elevated uric acid levels appeared to be a primary driver of changes in purine and pyrimidine biosynthesis. Alterations in Bcrp/Pgp dKO CSF levels of antioxidants, precursors of neurotransmitters, and uric acid suggests the transporters may contribute to the regulation of a healthy central nervous system in rats. Microbiome-generated metabolites were found to be elevated in dKO rat plasma and CSF. The altered dKO metabolome appeared to cause compensatory transcriptional change in urate biosynthesis and response to lipopolysaccharide in brain, oxidation-reduction processes and response to oxidative stress and porphyrin biosynthesis in kidney, and circadian rhythm genes in liver. These findings present insight into endogenous functions of Bcrp and Pgp, the impact that transporter substrates, inhibitors or polymorphisms may have on metabolism, how transporter inhibition could rewire drug sensitivity indirectly through metabolic changes, and identify functional Bcrp biomarkers.

EGR1 regulates cellular metabolism and survival in endocrine resistant breast cancer.

About 70% of all breast cancers are estrogen receptor alpha positive (ER+; ESR1). Many are treated with antiestrogens. Unfortunately, de novo and acquired resistance to antiestrogens is common but the underlying mechanisms remain unclear. Since growth of cancer cells is dependent on adequate energy and metabolites, the metabolomic profile of endocrine resistant breast cancers likely contains features that are deterministic of cell fate. Thus, we integrated data from metabolomic and transcriptomic analyses of ER+ MCF7-derived breast cancer cells that are antiestrogen sensitive (LCC1) or resistant (LCC9) that resulted in a gene-metabolite network associated with EGR1 (early growth response 1). In human ER+ breast tumors treated with endocrine therapy, higher EGR1 expression was associated with a more favorable prognosis. Mechanistic studies showed that knockdown of EGR1 inhibited cell growth in both cells and EGR1 overexpression did not affect antiestrogen sensitivity. Comparing metabolite profiles in LCC9 cells following perturbation of EGR1 showed interruption of lipid metabolism. Tolfenamic acid, an anti-inflammatory drug, decreased EGR1 protein levels and synergized with antiestrogens in inhibiting cell proliferation in LCC9 cells. Collectively, these findings indicate that EGR1 is an important regulator of breast cancer cell metabolism and is a promising target to prevent or reverse endocrine resistance.

Replicatively senescent human fibroblasts reveal a distinct intracellular metabolic profile with alterations in NAD+ and nicotinamide metabolism.

Cellular senescence occurs by proliferative exhaustion (PEsen) or following multiple cellular stresses but had not previously been subject to detailed metabolomic analysis. Therefore, we compared PEsen fibroblasts with proliferating and transiently growth arrested controls using a combination of different mass spectroscopy techniques. PEsen cells showed many specific alterations in both the NAD+ de novo and salvage pathways including striking accumulations of nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) in the amidated salvage pathway despite no increase in nicotinamide phosphoribosyl transferase or in the NR transport protein, CD73. Extracellular nicotinate was depleted and metabolites of the deamidated salvage pathway were reduced but intracellular NAD+ and nicotinamide were nevertheless maintained. However, sirtuin 1 was downregulated and so the accumulation of NMN and NR was best explained by reduced flux through the amidated arm of the NAD+ salvage pathway due to reduced sirtuin activity. PEsen cells also showed evidence of increased redox homeostasis and upregulated pathways used to generate energy and cellular membranes; these included nucleotide catabolism, membrane lipid breakdown and increased creatine metabolism. Thus PEsen cells upregulate several different pathways to sustain their survival which may serve as pharmacological targets for the elimination of senescent cells in age-related disease.

Oxidative Stress and Metabolic Perturbations in Wooden Breast Disorder in Chickens.

This study was conducted to characterize metabolic features of the breast muscle (pectoralis major) in chickens affected with the Wooden Breast myopathy. Live birds from two purebred chicken lines and one crossbred commercial broiler population were clinically examined by manual palpation of the breast muscle (pectoralis major) at 47-48 days of age. Metabolite abundance was determined by gas chromatography/mass spectrometry (GC/MS) and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) using breast muscle tissue samples from 16 affected and 16 unaffected chickens. Muscle glycogen content was also quantified in breast muscle tissue samples from affected and unaffected chickens. In total, levels of 140 biochemicals were significantly different (FDR<0.1 and fold-change A/U>1.3 or <0.77) between affected and unaffected chickens. Glycogen content measurements were considerably lower (1.7-fold) in samples taken from Wooden Breast affected birds when compared with samples from unaffected birds. Affected tissues exhibited biomarkers related to increased oxidative stress, elevated protein levels, muscle degradation, and altered glucose utilization. Affected muscle also showed elevated levels of hypoxanthine, xanthine, and urate molecules, the generation of which can contribute to altered redox homeostasis. In conclusion, our findings show that Wooden Breast affected tissues possess a unique metabolic signature. This unique profile may identify candidate biomarkers for diagnostic utilization and provide mechanistic insight into altered biochemical processes contributing to tissue hardening associated with the Wooden Breast myopathy in commercial chickens.

Transmissible microbial and metabolomic remodeling by soluble dietary fiber improves metabolic homeostasis.

Dietary fibers are increasingly appreciated as beneficial nutritional components. However, a requisite role of gut microbiota in fiber function and the overall impact of fibers on metabolomic flux remain unclear. We herein showed enhancing effects of a soluble resistant maltodextrin (RM) on glucose homeostasis in mouse metabolic disease models. Remarkably, fecal microbiota transplantation (FMT) caused pronounced and time-dependent improvement in glucose tolerance in RM recipient mice, indicating a causal relationship between microbial remodeling and metabolic efficacy. Microbial 16S sequencing revealed transmissible taxonomic changes correlated with improved metabolism, notably enrichment of probiotics and reduction of Alistipes and Bacteroides known to associate with high fat/protein diets. Metabolomic profiling further illustrated broad changes, including enrichment of phenylpropionates and decreases in key intermediates of glucose utilization, cholesterol biosynthesis and amino acid fermentation. These studies elucidate beneficial roles of RM-dependent microbial remodeling in metabolic homeostasis, and showcase prevalent health-promoting potentials of dietary fibers.

Women at extreme risk for obesity-related carcinogenesis: Baseline endometrial pathology and impact of bariatric surgery on weight, metabolic profiles and quality of life.

OBJECTIVES: The study objectives were to determine baseline endometrial histology in morbidly obese women undergoing bariatric surgery and to assess the surgical intervention's impact on serum metabolic parameters, quality of life (QOL), and weight. METHODS: Women undergoing bariatric surgery were enrolled. Demographic and clinicopathologic data, serum, and endometrium (if no prior hysterectomy) were collected preoperatively and serum collected postoperatively. Serum global biochemical data were assessed pre/postoperatively. Welch's two sample t-tests and paired t-tests were used to identify significant differences. RESULTS: Mean age of the 71 women enrolled was 44.2 years, mean body mass index (BMI) was 50.9 kg/m(2), and mean weight loss was 45.7 kg. Endometrial biopsy results: proliferative (13/30; 43%), insufficient (8/30; 27%), secretory (6/30; 20%) and hyperplasia (3/30; 10%-1 complex atypical, 2 simple). QOL data showed significant improvement in physical component scores (PCS means 33.9 vs. 47.2 before/after surgery; p<0.001). Twenty women underwent metabolic analysis which demonstrated significantly improved glucose homeostasis, improved insulin responsiveness, and free fatty acid levels. Significant perturbations in tryptophan, phenylalanine and heme metabolism suggested decreased inflammation and alterations in the intestinal microbiome. Most steroid hormones were not significantly impacted with the exception of decreased DHEAS and 4-androsten metabolites. CONCLUSIONS: Bariatric surgery is accompanied by an improved physical quality of life as well as beneficial changes in glucose homeostasis, insulin responsiveness, and inflammation to a greater extent than the hormonal milieu. The potential cancer protective effects of bariatric surgery may be due to other mechanisms other than simply hormonal changes.

Adaptive mitochondrial reprogramming and resistance to PI3K therapy.

BACKGROUND: Small molecule inhibitors of phosphatidylinositol-3 kinase (PI3K) have been developed as molecular therapy for cancer, but their efficacy in the clinic is modest, hampered by resistance mechanisms. METHODS: We studied the effect of PI3K therapy in patient-derived tumor organotypic cultures (from five patient samples), three glioblastoma (GBM) tumor cell lines, and an intracranial model of glioblastoma in immunocompromised mice (n = 4-5 mice per group). Mechanisms of therapy-induced tumor reprogramming were investigated in a global metabolomics screening, analysis of mitochondrial bioenergetics and cell death, and modulation of protein phosphorylation. A high-throughput drug screening was used to identify novel preclinical combination therapies with PI3K inhibitors, and combination synergy experiments were performed. All statistical methods were two-sided. RESULTS: PI3K therapy induces global metabolic reprogramming in tumors and promotes the recruitment of an active pool of the Ser/Thr kinase, Akt2 to mitochondria. In turn, mitochondrial Akt2 phosphorylates Ser31 in cyclophilin D (CypD), a regulator of organelle functions. Akt2-phosphorylated CypD supports mitochondrial bioenergetics and opposes tumor cell death, conferring resistance to PI3K therapy. The combination of a small-molecule antagonist of CypD protein folding currently in preclinical development, Gamitrinib, plus PI3K inhibitors (PI3Ki) reverses this adaptive response, produces synergistic anticancer activity by inducing mitochondrial apoptosis, and extends animal survival in a GBM model (vehicle: median survival = 28.5 days; Gamitrinib+PI3Ki: median survival = 40 days, P = .003), compared with single-agent treatment (PI3Ki: median survival = 32 days, P = .02; Gamitrinib: median survival = 35 days, P = .008 by two-sided unpaired t test). CONCLUSIONS: Small-molecule PI3K antagonists promote drug resistance by repurposing mitochondrial functions in bioenergetics and cell survival. Novel combination therapies that target mitochondrial adaptation can dramatically improve on the efficacy of PI3K therapy in the clinic.

Senescent human fibroblasts show increased glycolysis and redox homeostasis with extracellular metabolomes that overlap with those of irreparable DNA damage, aging, and disease.

Cellular senescence can modulate various pathologies and is associated with irreparable DNA double-strand breaks (IrrDSBs). Extracellular senescence metabolomes (ESMs) were generated from fibroblasts rendered senescent by proliferative exhaustion (PEsen) or 20 Gy of γ rays (IrrDSBsen) and compared with those of young proliferating cells, confluent cells, quiescent cells, and cells exposed to repairable levels of DNA damage to identify novel noninvasive markers of senescent cells. ESMs of PEsen and IrrDSBsen overlapped and showed increased levels of citrate, molecules involved in oxidative stress, a sterol, monohydroxylipids, tryptophan metabolism, phospholipid, and nucleotide catabolism, as well as reduced levels of dipeptides containing branched chain amino acids. The ESM overlaps with the aging and disease body fluid metabolomes, supporting their utility in the noninvasive detection of human senescent cells in vivo and by implication the detection of a variety of human pathologies. Intracellular metabolites of senescent cells showed a relative increase in glycolysis, gluconeogenesis, the pentose-phosphate pathway, and, consistent with this, pyruvate dehydrogenase kinase transcripts. In contrast, tricarboxylic acid cycle enzyme transcript levels were unchanged and their metabolites were depleted. These results are surprising because glycolysis antagonizes senescence entry but are consistent with established senescent cells entering a state of low oxidative stress.

Akt inhibition enhances expansion of potent tumor-specific lymphocytes with memory cell characteristics.

Adoptive cell therapy (ACT) using autologous tumor-infiltrating lymphocytes (TIL) results in complete regression of advanced cancer in some patients, but the efficacy of this potentially curative therapy may be limited by poor persistence of TIL after adoptive transfer. Pharmacologic inhibition of the serine/threonine kinase Akt has recently been shown to promote immunologic memory in virus-specific murine models, but whether this approach enhances features of memory (e.g., long-term persistence) in TIL that are characteristically exhausted and senescent is not established. Here, we show that pharmacologic inhibition of Akt enables expansion of TIL with the transcriptional, metabolic, and functional properties characteristic of memory T cells. Consequently, Akt inhibition results in enhanced persistence of TIL after adoptive transfer into an immunodeficient animal model and augments antitumor immunity of CD8 T cells in a mouse model of cell-based immunotherapy. Pharmacologic inhibition of Akt represents a novel immunometabolomic approach to enhance the persistence of antitumor T cells and improve the efficacy of cell-based immunotherapy for metastatic cancer.

De novo synthesize of bile acids in pulmonary arterial hypertension lung.

Although multiple, complex molecular studies have been done for understanding the development and progression of pulmonary hypertension (PAH), little is known about the metabolic heterogeneity of PAH. Using a combination of high-throughput liquid-and-gas-chromatography-based mass spectrometry, we found bile acid metabolites, which are normally product derivatives of the liver and gallbladder, were highly increased in the PAH lung. Microarray showed that the gene encoding cytochrome P450 7B1 (CYP7B1), an isozyme for bile acid synthesis, was highly expressed in the PAH lung compared with the control. CYP7B1 protein was found to be primarily localized on pulmonary vascular endothelial cells suggesting de novo bile acid synthesis may be involved in the development of PAH. Here, by profiling the metabolomic heterogeneity of the PAH lung, we reveal a newly discovered pathogenesis mechanism of PAH.

Large-scaled metabolic profiling of human dermal fibroblasts derived from pseudoxanthoma elasticum patients and healthy controls.

Mutations in the ABC transporter ABCC6 were recently identified as cause of Pseudoxanthoma elasticum (PXE), a rare genetic disorder characterized by progressive mineralization of elastic fibers. We used an untargeted metabolic approach to identify biochemical differences between human dermal fibroblasts from healthy controls and PXE patients in an attempt to find a link between ABCC6 deficiency, cellular metabolic alterations and disease pathogenesis. 358 compounds were identified by mass spectrometry covering lipids, amino acids, peptides, carbohydrates, nucleotides, vitamins and cofactors, xenobiotics and energy metabolites. We found substantial differences in glycerophospholipid composition, leucine dipeptides, and polypeptides as well as alterations in pantothenate and guanine metabolism to be significantly associated with PXE pathogenesis. These findings can be linked to extracellular matrix remodeling and increased oxidative stress, which reflect characteristic hallmarks of PXE. Our study could facilitate a better understanding of biochemical pathways involved in soft tissue mineralization.

Metabolomic profiles and childhood obesity.

OBJECTIVE: To identify metabolite patterns associated with childhood obesity, to examine relations of these patterns with measures of adiposity and cardiometabolic risk, and to evaluate associations with maternal peripartum characteristics. METHODS: Untargeted metabolomic profiling was used to quantify metabolites in plasma of 262 children (6-10 years). Principal components analysis was used to consolidate 345 metabolites into 18 factors and identified two that differed between obese (BMI ≥ 95‰; n = 84) and lean children (BMI < 85‰; n = 150). The relations of these factors with adiposity (fat mass, BMI, skinfold thicknesses) and cardiometabolic biomarkers (HOMA-IR, triglycerides, leptin, adiponectin, hsCRP, IL-6) using multivariable linear regression was then investigated. Finally, the associations of maternal prepregnancy obesity, gestational weight gain, and gestational glucose tolerance with the offspring metabolite patterns was examined. RESULTS: A branched-chain amino acid (BCAA)-related pattern and an androgen hormone pattern were higher in obese vs. lean children. Both patterns were associated with adiposity and worse cardiometabolic profiles. For example, each increment in the BCAA and androgen pattern scores corresponded with 6% (95% CI: 1, 13%) higher HOMA-IR. Children of obese mothers had 0.61 (0.13, 1.08) higher BCAA score than their counterparts. CONCLUSIONS: BCAA and androgen metabolites were associated with adiposity and cardiometabolic risk during mid-childhood. Maternal obesity may contribute to altered offspring BCAA metabolism.

The glucose transporter Glut1 is selectively essential for CD4 T cell activation and effector function.

CD4 T cell activation leads to proliferation and differentiation into effector (Teff) or regulatory (Treg) cells that mediate or control immunity. While each subset prefers distinct glycolytic or oxidative metabolic programs in vitro, requirements and mechanisms that control T cell glucose uptake and metabolism in vivo are uncertain. Despite expression of multiple glucose transporters, Glut1 deficiency selectively impaired metabolism and function of thymocytes and Teff. Resting T cells were normal until activated, when Glut1 deficiency prevented increased glucose uptake and glycolysis, growth, proliferation, and decreased Teff survival and differentiation. Importantly, Glut1 deficiency decreased Teff expansion and the ability to induce inflammatory disease in vivo. Treg cells, in contrast, were enriched in vivo and appeared functionally unaffected and able to suppress Teff, irrespective of Glut1 expression. These data show a selective in vivo requirement for Glut1 in metabolic reprogramming of CD4 T cell activation and Teff expansion and survival.

Increased cell surface free thiols identify effector CD8+ T cells undergoing T cell receptor stimulation.

Recognition of peptide Major Histocompatibility Complexes (MHC) by the T cell receptor causes rapid production of reactive oxygen intermediates (ROI) in naïve CD8(+) T cells. Because ROI such as H2O2 are membrane permeable, mechanisms must exist to prevent overoxidation of surface proteins. In this study we used fluorescently labeled conjugates of maleimide to measure the level of cell surface free thiols (CSFT) during the development, activation and differentiation of CD8(+) T cells. We found that during development CSFT were higher on CD8 SP compared to CD4 SP or CD4CD8 DP T cells. After activation CSFT became elevated prior to division but once proliferation started levels continued to rise. During acute viral infection CSFT levels were elevated on antigen-specific effector cells compared to memory cells. Additionally, the CSFT level was always higher on antigen-specific CD8(+) T cells in lymphoid compared to nonlymphoid organs. During chronic viral infection, CSFT levels were elevated for extended periods on antigen-specific effector CD8(+) T cells. Finally, CSFT levels on effector CD8(+) T cells, regardless of infection, identified cells undergoing TCR stimulation. Taken together these data suggest that CD8(+) T cells upregulate CSFT following receptor ligation and ROI production during infection to prevent overoxidation of surface proteins.

Landscape of the mitochondrial Hsp90 metabolome in tumours.

Reprogramming of tumour cell metabolism contributes to disease progression and resistance to therapy, but how this process is regulated on the molecular level is unclear. Here we report that heat shock protein 90-directed protein folding in mitochondria controls central metabolic networks in tumour cells, including the electron transport chain, citric acid cycle, fatty acid oxidation, amino acid synthesis and cellular redox status. Specifically, mitochondrial heat shock protein 90, but not cytosolic heat shock protein 90, binds and stabilizes the electron transport chain Complex II subunit succinate dehydrogenase-B, maintaining cellular respiration under low-nutrient conditions, and contributing to hypoxia-inducible factor-1α-mediated tumorigenesis in patients carrying succinate dehydrogenase-B mutations. Thus, heat shock protein 90-directed proteostasis in mitochondria regulates tumour cell metabolism, and may provide a tractable target for cancer therapy.

Metabolic regulation of T lymphocytes.

T cell activation leads to dramatic shifts in cell metabolism to protect against pathogens and to orchestrate the action of other immune cells. Quiescent T cells require predominantly ATP-generating processes, whereas proliferating effector T cells require high metabolic flux through growth-promoting pathways. Further, functionally distinct T cell subsets require distinct energetic and biosynthetic pathways to support their specific functional needs. Pathways that control immune cell function and metabolism are intimately linked, and changes in cell metabolism at both the cell and system levels have been shown to enhance or suppress specific T cell functions. As a result of these findings, cell metabolism is now appreciated as a key regulator of T cell function specification and fate. This review discusses the role of cellular metabolism in T cell development, activation, differentiation, and function to highlight the clinical relevance and opportunities for therapeutic interventions that may be used to disrupt immune pathogenesis.

Peroxiredoxin II regulates effector and secondary memory CD8+ T cell responses.

Reactive oxygen intermediates (ROI) generated in response to receptor stimulation play an important role in cellular responses. However, the effect of increased H(2)O(2) on an antigen-specific CD8(+) T cell response was unknown. Following T cell receptor (TCR) stimulation, the expression and oxidation of peroxiredoxin II (PrdxII), a critical antioxidant enzyme, increased in CD8(+) T cells. Deletion of PrdxII increased ROI, S phase entry, division, and death during in vitro division. During primary acute viral and bacterial infection, the number of effector CD8(+) T cells in PrdxII-deficient mice was increased, while the number of memory cells were similar to those of the wild-type cells. Adoptive transfer of P14 TCR transgenic cells demonstrated that the increased expansion of effector cells was T cell autonomous. After rechallenge, effector CD8(+) T cells in mutant animals were more skewed to memory phenotype than cells from wild-type mice, resulting in a larger secondary memory CD8(+) T cell pool. During chronic viral infection, increased antigen-specific CD8(+) T cells accumulated in the spleens of PrdxII mutant mice, causing mortality. These results demonstrate that PrdxII controls effector CD8(+) T cell expansion, secondary memory generation, and immunopathology.

Estrogen-related receptor-α is a metabolic regulator of effector T-cell activation and differentiation.

Stimulation of resting CD4(+) T lymphocytes leads to rapid proliferation and differentiation into effector (Teff) or inducible regulatory (Treg) subsets with specific functions to promote or suppress immunity. Importantly, Teff and Treg use distinct metabolic programs to support subset specification, survival, and function. Here, we describe that the orphan nuclear receptor estrogen-related receptor-α (ERRα) regulates metabolic pathways critical for Teff. Resting CD4(+) T cells expressed low levels of ERRα protein that increased on activation. ERRα deficiency reduced activated T-cell numbers in vivo and cytokine production in vitro but did not seem to modulate immunity through inhibition of activating signals or viability. Rather, ERRα broadly affected metabolic gene expression and glucose metabolism essential for Teff. In particular, up-regulation of Glut1 protein, glucose uptake, and mitochondrial processes were suppressed in activated ERRα(-/-) T cells and T cells treated with two chemically independent ERRα inhibitors or by shRNAi. Acute ERRα inhibition also blocked T-cell growth and proliferation. This defect appeared as a result of inadequate glucose metabolism, because provision of lipids, but not increased glucose uptake or pyruvate, rescued ATP levels and cell division. Additionally, we have shown that Treg requires lipid oxidation, whereas Teff uses glucose metabolism, and lipid addition selectively restored Treg--but not Teff--generation after acute ERRα inhibition. Furthermore, in vivo inhibition of ERRα reduced T-cell proliferation and Teff generation in both immunization and experimental autoimmune encephalomyelitis models. Thus, ERRα is a selective transcriptional regulator of Teff metabolism that may provide a metabolic means to modulate immunity.

The oxysterol, 27-hydroxycholesterol, links cholesterol metabolism to bone homeostasis through its actions on the estrogen and liver X receptors.

Osteoporosis and age-related bone loss are important public health concerns. Therefore, there is a high level of interest in the development of medical interventions and lifestyle changes that reduce the incidence of osteoporosis and age-related bone loss. Decreased bone mineral density is associated with high cholesterol, and patients on statins have increased bone mineral densities, strongly implicating cholesterol as a negative regulator of bone homeostasis. In this study, using both molecular and pharmacological approaches, we have been able to demonstrate that the primary cholesterol metabolite, 27-hydroxycholesterol, through its actions on both estrogen receptors and liver X receptors, decreases osteoblast differentiation and enhances osteoclastogenesis, resulting in increased bone resorbtion in mice. Induction of the short heterodimer partner protein by estrogens in osteoblasts can attenuate the liver X receptor-mediated actions of 27-hydroxycholesterol in bone. These data establish a mechanistic link between cholesterol and bone quality, highlight an unexpected target of estrogens in osteoblasts, and define a signaling axis, the therapeutic exploitation of which is likely to yield novel antiosteoporotic drugs.